The present invention relates generally to a bicycle crank set assembly, and more particularly to a two-piece, tubular, continuous and tapered crank set combining high strength with light weight.
In bicycle racing, the weight, strength, and rigidity of bicycle components are of ultimate importance to bicycle enthusiasts. Lighter components (especially the rotating parts) having superior strength and rigidity are much sought after.
Information relevant to attempts to address these component characteristics can be found in the following U.S. Pat. No. 5,493,937 to Edwards; U.S. Pat. No. 4,704,919 to Durham; and U.S. Pat. No. 5,924,336 to Richardson. However, each of these references suffers from one or more of the disadvantages described below. Therefore, these references do not effectively combine maximum load bearing capability with minimum weight.
Many existing bicycle crank sets include a two-piece crank axle having intricately joined crank arms. The crank arms are often separate members mechanically attached to the crank axle portions and sometimes reinforced with gussets. The mechanical attachments and reinforcing gussets undesirably result in a build up of weight at the attachment locations.
The present invention solves this problem by providing a continuous tubular member of monolithic design, combining a one-half crank axle portion with a crank arm. This design avoids weighty joint and joint reinforcements, efficiently providing superior strength and rigidity while minimizing weight.
Many existing crank sets include crank arms having a constant cross-section along the crank arm length. Since bearing loads addressed by crank arms are not constant over the crank arm length, a crank arm having a constant cross-section does not offer superior load carrying capabilities in relation to weight, thus being detrimental to achieving a maximum strength and rigidity to weight relationship.
The present invention provides tapered crank arms, analyzed for load bearing capacity at each point along the crank arm length. This design efficiently provides the minimal cross-section necessary, at each point along the crank arm length, to accommodate the loads faced at each point along the crank arm length. The monolithic and continuous design distributes all stresses uniformly and equally over each piece of the crank set.
Some existing crank sets join two crank axle portions within a bracket shell with a spline joint. Due to the nature of cyclical loads bearing on the crank axle, the weakest portion of the crank axle occurs at its smallest diameter, or at a stress concentration point. Since a spline joint joins one smaller diameter crank axle portion within another larger diameter crank axle portion, the overall crank axle bearing capacity is limited to its smaller diameter portion. Considering the limited dimensions of the bracket shell, the spline joint does not offer maximum load bearing capacity relative to overall crank axle diameter.
The present invention provides a uniform crank axle diameter of tubular design, efficiently maximizing load bearing capacity relative to crank axle diameter.
Existing crank sets having larger diameter crank axles with excessively reinforced and weighty couplings further require larger and heavier bearing sets or bearing set placement outside the bracket shell (as in Richardson). Since the boundary conditions of the bracket shell are fixed, larger bearing sets result in less space to accommodate large diameter crank axles. Therefore, there exists a need for bearing sets made of higher strength materials, so that smaller bearing sets could accommodate the crank axle loads. These smaller bearing sets would offer more space within a typical bracket shell for larger diameter crank axles, thus increasing crank axle bearing capacity without resorting to bearing set placement outside the bracket shell, which has the disadvantage of widening the crank set assembly.
The present invention solves this problem by providing high strength bearing sets of a diameter substantially smaller than that in existing crank sets, thus accommodating crank axles of larger diameter within the fixed diameter of the bracket shell and providing a crank set of superior load bearing capacity.
The present invention is a directed to a two-piece bicycle crank set combining superior strength and rigidity with light weight. The crank set comprises two thin-wall tubular members. The tubular members include crank axle portions of relatively large diameter and light weight, tapered crank arms where torsional, bending and shear loads determine the tubular cross sectional size at each location along the crank arm length and a reinforcing insert at the distal end of the crank arm to accommodate a variety of pedal axles, or a tubular pedal axle incorporated within the continuous, monolithic crank axle and crank arm tubular member.
In one aspect of the present invention, the crank set is significantly lighter than existing crank set assemblies and has superior load bearing capability. The crank set is designed with torsional, bending and shear loads determining the dimensions of the tubular crank axle and the tapered crank arm cross-section. The tubular members are designed and shaped to distribute all stresses uniformly and equally over the continuous tubular member.
In one aspect of the present invention, the crank set includes bearing sets having ultra high strength ball bearings allowing for smaller bearing sets than those found in existing crank set assemblies, therefore providing additional space within the bracket shell for a larger diameter crank axle.
In another aspect of the present invention, the crank set is split midway along its crank axle, with each crank set piece containing precisely one-half of the total crank axle length. The two, one-half crank axle portions are coupled within the bracket shell precisely midway between the two bearing sets. Connecting the crank axle portions precisely midway between the two bearing sets effectively eliminates all shear loads.
In another aspect of the present invention, the bicycle crank set comprises a first tubular member, a second tubular member, a spider connected to the second tubular member, and a coupling securing the first tubular member to the second tubular member within a bracket shell. The first and the second tubular members include a crank arm and a portion of a crank axle forming a continuous and monolithic thin-wall tubular shape.
In another aspect of the present invention, the first and the second tubular members further include a pedal axle, where the crank arm, the portion of the crank axle and the pedal axle all form a continuous and monolithic thin-wall tubular shape. In another of its aspects, a spider is further incorporated into the continuous, thin-wall tubular shape of the second tubular member.
In another aspect of the present invention, the first and the second tubular members are injection molded and made of aramid fiber composite material. Alternatively, the first and the second tubular members could be injection molded and made of carbon and glass fiber composite material. Further, the first and the second tubular member could be steel stamped parts of clamshell design and electron beam or laser welded together to form the continuous and monolithic thin-wall tubular shape.
In another aspect of the present invention, the coupling includes an outer sleeve, an inner sleeve, two exteriorly tapered and internally threaded bushings, and a threaded stud. Turning the stud threadably positions the bushings to expand the inner sleeve, securing the crank axle portions of the first and the second tubular members between the inner and the outer sleeves of the coupling.
In another aspect of the present invention, the coupling includes a mortise member and a tenon member. The tenon member fits into the mortise member to secure the crank axle portion of the first tubular member to that of the second tubular member. This coupling can further include an attachment bolt that passes through a clearance hole in one of the tubular members and threadably engages an attachment hole in the other tubular member. Threading the attachment bolt into the attachment hole securely interlocks the tenon member into the mortise member. The mortise and the tenon members can be either tapered or non-tapered. This coupling can further include separation bolt, where removing the attachment bolt and threading the separation bolt into and through the clearance hole forces the tenon member apart and away from the mortise member.
In another aspect of the present invention, the clearance hole has a diameter of 10 mm and the attachment hole has a diameter of 8 mm. The diameter of the attachment bolt can be 8 mm and the diameter of the separation bolt can be 10 mm.
In another aspect of the present invention, the tenon and the mortise members are made of boron composites. Alternatively, the tenon and the mortise members can be made from high strength metal materials.
In another aspect of the present invention, the crank set further includes two bearing sets and the coupling within the bracket shell is located precisely midway between the two bearing sets. The bearing sets can include an outer cup, an inner cup, seals and ceramic balls. The ceramic balls can housed within the outer cup, the inner cup and the seals and can be separated from one another by retainers. The outer cup is in communication with the bracket shell and the inner cup is in communication with the crank axle portion of one of the tubular members. The ceramic balls can be made of silicon nitride. The outer and the inner cups can be made of 52100 steel and can be hardened and sputer coated with titanium aluminum nitride to provide an overall hardness exceeding Rockwell 90. The seals can be spring loaded Teflon garter. The retainers can be made of mylar, nylon, Delrin or an adequate engineering plastic.
In another aspect of the present invention, the crank arm further includes an internally threaded titanium insert capable of housing a variety of pedals. The crank arm can include a compression molded solid carbon fiber and glass composite portion, in the vicinity of the titanium insert, to house the titanium insert.